It is known that carbon-carbon double bonds in polymers may be successfully hydrogenated by treating the polymer in an organic solution with hydrogen in the presence of a catalyst, as disclosed in U.S. Pat. Nos. 6,881,797, 6,683,136, 6,410,657, 6,020,439, 5,705,571, 5,057,581, 5,652,191, 5,399,632, Journal of Polymer Science Part A: Polymer Chemistry Vol 30, no. 3, 1992, 471-484, U.S. Pat. Nos. 5,164,457 and 3,454,644. Such processes can be selective in the double bonds which are hydrogenated so that, for example, the double bonds in aromatic or naphthenic groups are not hydrogenated and double or triple bonds between carbon and other atoms such as nitrogen or oxygen are not affected. This field of art contains many examples of catalysts suitable for such hydrogenations, including catalysts based on cobalt, nickel, rhodium, ruthenium, osmium, and palladium. The suitability of the catalyst depends on the extent of hydrogenation required, the rate of the hydrogenation reaction and the presence or absence of other groups, such as carboxyl and nitrile groups, in the polymers. As a characteristic, such hydrogenation processes always require the use of a large amount of organic solvent to dissolve the polymers to be hydrogenated and work at a relatively high temperature (larger than 100° C.), which raises concerns with respect to the production cost and environmental protection. In this field of art it is furtheron desired to provide very efficient catalysts as this allows to minimize the required catalyst amount, which can milden the environmental concerns and reduce production cost.
US-2006/0211826 A1 discloses a process for the selective hydrogenation of olefinic unsaturation in polymers and copolymers in bulk form without the addition of any organic solvent, in the presence of hydrogen and a rhodium based catalyst. However, such process generally needs to be performed at relatively high temperatures of well above 100° C.
U.S. Pat. Nos. 6,110,397, 6,063,307 and 5,837,158 disclose a process for removing hydrogen from the atmosphere within enclosed spaces by using a hydrogen getter. The hydrogen getter comprises organic polymer molecules having carbon-carbon double bounds, and a hydrogenation catalyst consisting of a noble metal catalyst such as palladium or platinum. Such hydrogenation catalyst is mandatorily supported on an inert catalyst support material. The C═C double bonds are hydrogenated during the removal of hydrogen. The processes of U.S. Pat. Nos. 6,110,397, 6,063,307 and 5,837,158 do not have the aim to provide maximum hydrogenation degrees but a viable way to remove undesired hydrogen traces from the environment.
As efficient catalysts, iridium based catalysts, have been described in Platinum Metals Review, Vol 22, no. 4, 1978, 126-129 and Accounts of Chemical Research, Vol 12, 1979, 331-337) and received attention due to their high activity in hydrogenations even with low-molecular weight olefins even if those are highly substituted.
US-2007/0155909 A1 discloses a metal catalyst for hydrogenating unsaturated carbon-carbon bonds of copolymers wherein the said copolymers are dissolved in a solvent which is typically organic. The metal catalyst is a bimetallic complex comprising iridium and ruthenium.
In “Chemical Industries”, Vol. 104 (Catalysis of Organic Reactions), 125-134, 2005, investigations about the use of Crabtree's catalyst for the hydrogenation of nitrile butadiene rubber in an organic solution are described. The catalyst was efficient for selective hydrogenation of olefinic groups in nitrile butadiene rubber at 120-140° C. and a hydrogen pressure of few hundred psi.
To eliminate the requirement for a large amount of organic solvents in hydrogenation operation, Laura R. Gilliom (Macromolecules Vol 22, no. 2, 1989, 662-665) and Laura R. Gilliom and Kevin G. Honnell (Macromolecules Vol. 25, no. 22, 1992, 6066-6068) had attempted to realize bulk hydrogenation of polymers using iridium based catalysts [Ir(COD)(py)(tcyp)]PF6 and [Ir(COD)(PMePh2)2]PF6 (COD=1,5-cyclooctadiene, Py=pyridine, tcyp=tricyclohexylphosphine). In Macromolecules Vol 22, no. 2, 1989, 662-665 [Ir(COD)(PMePh2)2]PF6 is used for hydrogenating thermoplastic polymers and compared with Wilkinson's catalyst, i.e. Rh(PPh3)3Cl. The samples for the hydrogenation studies were cast from solution. The polymer was first dissolved in toluene or dichloromethane, the catalyst added and the solvent then removed. Hydrogenation rates of 80% have been achieved, however, the reaction rate was very low and the reaction needed five or even more than five days to reach a relatively high conversion. Additionally it was recognized that only high catalyst loadings result in a sufficient hydrogen uptake in the first five reaction hours. Examples with different catalyst loadings show that a loading with 1 weight % catalyst does not result in any recognizable hydrogen uptake. Using a catalyst loading of 3.2 weight % improves the hydrogen uptake, but only with 9.1 weight % is an acceptable hydrogen uptake recognized. In Macromolecules Vol. 25, no. 22, 1992, 6066-6068 [Ir(COD)(py)(tcyp)]PF6 is used as catalyst.
In summary, the research in this area, namely, the hydrogenation of diene-based polymers, has been very successful if the polymers were dissolved in an organic solvent or if the hydrogenation is carried out at a relatively high reaction temperature. The research activities clearly focused on efficient hydrogenation of diene-based polymers in bulk form, however, are so far very limited: In Macromolecules Vol 22, no. 2, 1989, 662-665 and Macromolecules Vol 25, no. 22, 1992, 6066-6068 rhodium and iridium based catalysts were applied, however, the reaction rate was very low and the amount of iridium based catalyst to be used was very high.
The present invention therefore has the object to provide a new and improved process allowing the selective hydrogenation of a diene-based polymer in bulk form with a high degree of hydrogenation within acceptable short reaction time and at mild reaction temperatures.